1. Field of the Invention
The invention relates to improvements in an electric motor; and more particularly, to an electric motor having an external rotor.
2. Description of the Related Art
Electric motors are commonly used in many different commercial and residential applications. An electric motor typically comprises a rotor and a stator, each of the rotor and stator having multiple magnets disposed about the periphery. The poles of the magnets on the rotor and the poles of the magnets on the stator are controlled such that the poles of the rotor are drawn to or repulsed from a corresponding pole on the stator to effect the rotation of the rotor relative to the stator. The control of the poles is normally accomplished by at least one of the series of magnets on either the rotor or stator being made from an electromagnetic winding whose polarity can be altered by changing the direction of current passing through the winding.
In most electric motors, the stator is part of or forms the external housing of the electric motor and the rotor comprises a shaft mounted within the stator for relative rotation therein. However, in some applications, it is desirable for the rotor to be on the outside of the motor and the stator to be on the inside. This arrangement is sometimes called a squirrel cage motor. Most often, they are driven with brushless commutators and DC power, so they are also often called brushless DC motors (BLDC's). An electric motor having an external rotor is typically used to drive belts and the like while being positioned within the interior of the belt. A suitable application for such a configuration would be a materials handling environment or a treadmill.
Several problems are attendant to squirrel cage motors in materials handling applications. One difficulty associated with electric motors with external rotors is that power output is normally capped so that only short material handling runs can be driven by a single motor and only relatively light weight articles can be propelled by the motor. Greater output power from an electric motor is typically achieved by increasing the size of the components. But for a squirrel cage motor, increased size is impractical. A larger diameter stator is undesirable because of added weight and the balance condition of the rotor. A longer length of the stator can result in deflection of the shaft mounting the stator in response to the magnetic attraction between the rotor and the stator causing the rotor and stator to contact, reducing the motor's performance or, in extreme cases, prohibiting motor rotation altogether.
Another difficulty is cooling the motor, especially at higher speeds or torques. At light loads or low speeds, cooling is not a problem, but with demands for such motors having more power output and higher speeds, the need for transferring heat away from the rotating parts becomes apparent. Most small squirrel cage motors in material handling applications are cooled by internal oil, which creates seal problems.
Other problems with such motors include noise, assembly, and accurately controlling commutation for smooth operation at low speeds and changes in speeds under torque. With higher torque output, especially at higher speeds, vibrations and consequent noise can become unacceptable. Moreover, it is known to determine the position of the rotor relative to the stator by means of a Hall effect sensor. But it has been found that the accuracy of this method for controlling fine changes in speed or torque is unacceptable. Yet further, the length of such motors is limited by their structures. For applications where wider belts are needed, e.g., treadmills, such a motor cannot effectively be a drive roller because it is not long enough.